Chlorine dioxide for use as a pulp mill bleach chemical is produced by reduction of sodium chlorate in an aqueous acid reaction medium. It is well known that the reaction to produce chlorine dioxide is represented by the equation: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O
The chloride ion for this reaction may be provided from an external source, in which case chlorine is co-produced with the chlorine dioxide, or may be provided in situ by reduction of the co-produced chlorine using reducing agents, such as methanol and sulfur dioxide.
One well-known commercial embodiment of the process is the so-called ERCO R3 (trademarks) process, wherein sodium chlorate, sodium chloride and sulfuric acid are reacted together at an acidity of about 2 to about 4.8 normal in a single vessel at the boiling point of the reaction medium while a subatmospheric pressure is applied to the reactor. The chlorine dioxide and chlorine are removed from the reaction vessel in admixture with the steam and anhydrous neutral sodium sulfate precipitates from the reaction medium, once saturation has been achieved after start-up. This process is generally described in U.S. Pat. No. 3,864,456 (E26), assigned to the assignee hereof.
As described in U.S. Pat. No. 3,895,100 (E107), assigned to the assignee hereof, a slurry of reaction liquor and sodium sulphate crystals is removed from the reaction vessel, the crystals are removed by filtration, make-up sodium chlorate and sodium chloride are added and the resulting recycle stream is reheated while being subjected to a back pressure sufficient to prevent it from boiling in the reboiler. The reheated recycle liquor passes through the throat of a venturi where make-up sulfuric acid is introduced. Chlorine dioxide production immediately commences and the recycle mixture is allowed to expand on the downstream side of the venturi and the resulting three-phase mixture of gases, solids and liquid is re-introduced to the reaction vessel. Chlorine dioxide and chlorine in the recycle mixture and produced in the reaction vessel are removed from the reaction vessel.
More recently, the ERCO R8 (trademark) process has become popular with pulp mills. This process differs from the R3 process in using a much higher acidity, generally greater than about 9 normal, and methanol in place of the sodium chloride. In addition, no catalyst is required. The result of the changes is that substantially pure chlorine dioxide is formed (the methanol reduces the co-produced chlorine to chloride ions) at greater efficiency than is achievable in the R3 process even with a catalyst. The R8 process is described in U.S. Pat. Nos. 4,081,520 (E130), 4,393,035 (E170) and 4,393,036 (E172), assigned to the assignee hereof.
One drawback of the R8 process, however, is that, because of the significantly higher acidity that is used than in the R3 process, the sodium sulphate by-product is acidic and requires neutralization.